Lithium secondary battery comprising electrolyte

ABSTRACT

The present invention relates to a lithium secondary battery. The lithium secondary battery includes a positive electrode including a positive active material; a negative electrode including a negative active material; and an electrolyte including a non-aqueous organic solvent, a lithium salt, and an additive including a compound represented by Chemical Formula 1. The negative active material includes Si at about 0.1 wt % to about 32 wt % in amount based on a total weight of the negative active material. 
     
       
         
         
             
             
         
       
         
         
           
             wherein, in Chemical Formula 1, A is a substituted or unsubstituted aliphatic chain or (—C 2 H 4 —O—C 2 H 4 —)n, and n is an integer from 1 to 10.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation-in-part of U.S. patent applicationSer. No. 16/335,240, filed on Mar. 20, 2019, which is a National PhasePatent Application of International Patent Application NumberPCT/KR2017/009934, filed on Sep. 11, 2017, which claims priority ofKorean Patent Application No. 10-2016-0126806, filed on Sep. 30, 2016.The entire contents of all of which are incorporated herein byreference.

TECHNICAL FIELD

This disclosure relates to a lithium secondary battery.

BACKGROUND ART

A portable information device such as a cell phone, a laptop, a smartphone, and the like or an electric vehicle has used a lithium secondarybattery having high energy density and easy portability as a drivingpower source.

In general, a lithium secondary battery is manufactured by usingmaterials capable of reversibly intercalating and deintercalatinglithium ions as a positive active material and a negative activematerial and filling an electrolyte between the positive electrode andthe negative electrode.

Lithium-transition metal oxides are used as the positive active materialof the lithium secondary battery, various types of carbon-basedmaterials are used as the negative active material, and lithium saltsdissolved in the non-aqueous organic solvent are used as an electrolyte.

In particular, as a lithium secondary battery exhibits batterycharacteristics by complex reactions such as those between a positiveelectrode and an electrolyte, a negative electrode and an electrolyte,and the like, the use of a suitable electrolyte is one of importantparameters for improving the performance of a lithium secondary battery.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

Embodiments provide a lithium secondary battery exhibiting excellentstorage safety while maintaining good cycle-life characteristics.

Technical Solution

In one aspect, the present disclosure provides a lithium secondarybattery including: a positive electrode including a positive activematerial; a negative electrode including a negative active material; andan electrolyte including a non-aqueous organic solvent, a lithium salt,and an additive including a compound represented by Chemical Formula 1,wherein the negative active material includes Si at about 0.1 wt % toabout 32 wt % in amount based on a total weight (i.e., 100 wt %) of thenegative active material.

In Chemical Formula 1, A is a substituted or unsubstituted aliphaticchain or (—C₂H₄—O—C₂H₄—)n, and n is an integer from 1 to 10.

Advantageous Effects

According to the embodiments, while cycle-life characteristics aremaintained, the storage safety of a lithium secondary battery may beimproved.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a lithium secondary battery according toan embodiment of the present disclosure.

FIG. 2 shows CV characteristic evaluation results of Example 1.

FIG. 3 shows CV characteristic evaluation results of Comparative Example1.

FIG. 4A shows LSV evaluation results of Example 1 and ComparativeExample 1.

FIG. 4B is an enlarged view showing a y-axis scale of FIG. 4A.

FIG. 5 shows the results of measurement of cycle characteristics forExample 2 and Comparative Example 2.

MODE FOR INVENTION

Hereinafter, the present disclosure will be described more fullyhereinafter with reference to the accompanying drawings, in whichexemplary embodiments of the invention are shown. However, thisdisclosure may be embodied in many different forms and is not construedas limited to the example embodiments set forth herein.

In order to clearly illustrate the present invention, parts that are notrelated to the description are omitted, and the same or similarcomponents are denoted by the same reference numerals throughout thespecification.

Sizes and thicknesses of components in the drawings are arbitrarilyexpressed for convenience of description and, thus, the presentinvention is not limited by the drawings.

In addition, unless explicitly described to the contrary, the word“comprise” and variations such as “comprises” or “comprising,” will beunderstood to imply the inclusion of stated elements but not theexclusion of any other elements.

An electrolyte of a lithium secondary battery may be generally anorganic solvent in which a lithium salt is dissolved. In particular,non-aqueous organic solvents having a high ionic conductivity anddielectric constant and a low viscosity may be used.

In lithium secondary battery using a carbonate-based solvent of such anon-aqueous organic solvent, an irreversible side reaction may occurbetween the electrolyte and the positive electrode, and between theelectrolyte and the negative electrode at a high temperature and a highpressure.

The decomposition products produced by such a side reaction form a thickpassivation film which acts as a resistance on the surface of theelectrode and decrease a cycle life and capacity of the lithiumsecondary battery. Also, due to the decomposition of a carbonate-basedorganic solvent, gas is generated inside the battery which causes aswelling phenomenon and it may lead to battery explosion.

A lithium secondary battery according to one embodiment includes, as anelectrolyte for a lithium secondary battery for preventing or reducingthe swelling phenomenon of a lithium secondary battery and havingexcellent stability and cycle-life characteristics, an electrolyteincluding a compound represented by Chemical Formula 1 as an additive.

That is, the lithium secondary battery according to one embodimentincludes a positive electrode including a positive active material, anegative electrode including a negative active material, and anelectrolyte including a non-aqueous organic solvent, a lithium salt, andan additive including a compound represented by Chemical Formula 1.Furthermore, the negative active material includes Si at about 0.1 wt %to about 32 wt % in amount based on the total weight of the negativeactive material (i.e., based on 100 weight percent of the negativeactive material).

In Chemical Formula 1, A is a substituted or unsubstituted aliphaticchain or (—C₂H₄—O—C₂H₄-)n, and n is an integer from 1 to 10.

In Chemical Formula 1, A may be a C₂ to C₂₀ hydrocarbon chain or(—C₂H₄—O—C₂H₄-)n, and n may be an integer from 1 to 5.

In addition, the compound represented by Chemical Formula 1 may be acompound represented by Chemical Formula 1-1.

The negative active material includes Si at an amount of about 0.1 wt %to about 32 wt % based on 100 wt % of the total weight of the negativeactive material, and such a negative active material may be: i) amaterial including a core including a mixture of crystalline carbon andSi, and an amorphous carbon coating layer surrounding the core; or ii) amaterial including a core including crystalline carbon and Si adhered toa surface of the core.

As described above, the lithium secondary battery according to oneembodiment includes a negative electrode including a negative activematerial with Si at an amount of about 0.1 wt % to about 32 wt % basedon 100 wt % of the total weight of the negative active material, and anadditive including a compound represented by Chemical Formula 1. Whenthe electrolyte including the additive including the compoundrepresented by Chemical Formula 1 is applied to the lithium secondarybattery including the negative active material with Si at the rangeabove (e.g., at the specific amount), the cycle-life characteristics ofthe lithium secondary battery may be improved and the effect forreducing the generation of gas at a high temperature may be enhanced ormaximized and the resistance increase (e.g., increase in resistancevalue) during high-temperature storage may be greatly reduced, while theimprovements in the cycle-life characteristics of the lithium secondarybattery may be maintained. This is because the Si-containing (i.e.,Si-included) negative active material increases the capacity of thebattery, but the amount of the gas generation during high-temperaturestorage may be large and the resistance increase during high-temperaturestorage may also be large. However, the effects derived from theadditive of Chemical Formula 1, including reducing the amount of gasgenerated at a high temperature and reducing the resistance increaseduring high-temperature storage, may be obtained (e.g., largelyobtained). Thus, the use of the electrolyte with the additive ofChemical Formula 1 allows the lithium secondary battery to exhibit highcapacity owing to the Si-included negative active material and to havethe improved storage safety, especially, to have the improvedhigh-temperature storage safety and the reduced high storage resistanceincrease. That is, the use of the electrolyte with the additive ofChemical Formula 1 in combination with the Si-containing negative activematerial allows the lithium secondary battery to exhibit both highcapacity (due to the Si-containing negative active material) and theimproved storage safety, especially, the improved high-temperaturestorage safety, and the reduced high storage resistance increase.Furthermore, the effects related to the improvement duringhigh-temperature storage safety and the decrease in the high-temperatureresistance increase may be obtained in the lithium secondary batteryincluding a negative active material with Si at an amount of about 0.1wt % to about 32 wt %. Even if the electrolyte including the compound ofChemical Formula 1 as the additive is used, if a negative activematerial with Si at an amount of more than about 32 wt % is used, theamount of gas generation in high-temperature storage is large (e.g.,enlarged) and the resistance increase is high so that it is notdesirable. In addition, the effects for improving the high-temperaturestorage safety and reducing the high-temperature resistance increase maybe enhanced (e.g., at a maximum) for the lithium secondary batteryincluding a negative active material with Si at an amount of about 0.1wt % to about 32 wt % and an electrolyte with an additive of ChemicalFormula 1 at an amount of about 0.1 wt % to about 3 wt %.

Without being bound to any particular theory, the effects in which thecompound of Chemical Formula 1 reduces the amount of the gas generationmay be that the compound represented by Chemical Formula 1 includes adifluorophosphite (—OPF₂) group having excellent electrochemicalreactivity at both terminals. That is, the compound represented byChemical Formula 1 includes the difluorophosphite (—OPF₂) group withexcellent electrochemical reactivity at both terminals, which leads tothe reduced amount of gas generation.

During the initial charge of a lithium secondary battery, lithium ions,which are released from the lithium-transition metal oxide, i.e., thepositive active material, are transported into a negative electrode andintercalated thereinto. Because of its high reactivity, lithium reactswith the negative electrode to produce Li₂CO₃, Li₂O, LiOH, etc., therebyforming a thin film on the surface of the negative electrode. This thinfilm is referred to as a solid electrolyte interface (SEI) film. The SEIthin film formed during the initial charge prevents the reaction betweenlithium ions and the negative electrode or other materials during chargeand discharge. In addition, it also acts as an ion tunnel, allowing thepassage of only lithium ions. The ion tunnel prevents disintegration ofthe structure of the negative electrode, which is caused byco-intercalation of organic solvents of the electrolyte, having a highmolecular weight along with solvated lithium ions into the negativeelectrode. Once the SEI thin film is formed, lithium ions do not reactagain with the negative electrode or other materials, such that theamount of lithium ions is reversibly maintained. Therefore, in order toimprove the high-temperature cycle characteristics and thelow-temperature output of the lithium secondary battery, a rigid SEIthin film must be always formed on the negative electrode of the lithiumsecondary battery.

However, when the additive including the compound represented byChemical Formula 1 is included like (e.g., as in) the electrolyte for alithium secondary battery according to the present disclosure, a rigidSEI film having good ion conductivity is formed on the surface of thenegative electrode, and thereby it is possible to suppress adecomposition of the surface of the negative electrode during hightemperature cycle operation and to prevent an oxidation reaction of theelectrolyte solution.

When the compound represented by Chemical Formula 1 is decomposed, adifluorophosphite (—OPF₂) group and an ethylene dioxide fragment may beformed.

The difluorophosphite (—OPF₂) group may form a donor-acceptor bond withtransition metal oxide that is exposed on the surface of the positiveactive material due to excellent electrochemical reactivity and thus aprotective layer in a form of a composite may be formed.

In addition, the difluorophosphite (—OPF₂) adhered to the transitionmetal oxide at the initial charge of the lithium secondary battery maybe oxidized to a plurality of fluorophosphates, and thus a more stableinactive layer having excellent ion conductivity may be formed on thepositive electrode. Therefore, it is possible to prevent othercomponents of the electrolyte from being oxidation-decomposed, and as aresult, the cycle-life performance of the lithium secondary battery maybe improved and a swelling phenomenon may be prevented from occurring.

Further, the compound represented by Chemical Formula 1 and its oxideparticipate in the electrochemical reaction with the components of theSEI thin film to make the SEI thin film more rigid and to improvestability of other components included in the electrolyte by oxidativedecomposition.

In addition, the compound represented by Chemical Formula 1 forms acomposite with LiPF₆ and thus undesirable side reactions may beprevented from occurring, and it is possible to improve cycle-lifecharacteristics of the lithium secondary battery and to prevent thegeneration of gas in the lithium secondary battery, therebysignificantly reducing an occurrence rate of defects due to a swellingphenomenon.

On the other hand, the additive including the compound represented byChemical Formula 1 may be included in an amount of 0.1 wt % to 3 wt %based on a total amount of the electrolyte for a lithium secondarybattery. More specifically, the amount of the compound represented byChemical Formula 1 may be 0.1 wt % to 2 wt % or 0.15 wt % to 2 wt %.When the amount of the additive satisfies these ranges, a resistanceincrease may be prevented, and thus a lithium secondary battery havingimproved cycle-life characteristics may be realized.

The additive for a lithium secondary battery of this disclosure mayfurther include an additional additive. The additional additive may be,for example, at least one selected from the group consisting offluoroethylene carbonate, vinylene carbonate, vinyl ethylene carbonate,succinonitrile, hexane tricyanide, lithium tetrafluoroborate, andpropane sultone, but is not limited thereto.

Herein, an amount of the additional additive may be 0.1 wt % to 20 wt %based on a total amount of the electrolyte for a lithium secondarybattery. More specifically, the amount of the additional additive may be0.1 wt % to 15 wt %. When the amount of the additional additivesatisfies these ranges, battery resistance may be effectively suppressedand a lithium secondary battery having suitable cycle-lifecharacteristics may be realized.

On the other hand, the non-aqueous organic solvent serves as a mediumfor transporting ions taking part in the electrochemical reaction of alithium secondary battery.

The non-aqueous organic solvent may include a carbonate-based,ester-based, ether-based, ketone-based, alcohol-based, or aproticsolvent.

The carbonate-based solvent may include dimethyl carbonate (DMC),diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropylcarbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate(MEC), ethylene carbonate (EC), propylene carbonate (PC), butylenecarbonate (BC), and/or the like. The ester-based solvent may includemethyl acetate, ethyl acetate, n-propyl acetate, dimethylacetate,methylpropionate, ethyl propionate, propyl propionate, γ-butyrolactone,decanolide, valerolactone, mevalonolactone, caprolactone, and/or thelike.

The ether-based solvent may include dibutyl ether, tetraglyme, diglyme,dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and/or thelike, and the ketone-based solvent may include cyclohexanone, and/or thelike.

The alcohol-based solvent may include ethanol, isopropyl alcohol, and/orthe like, and the aprotic solvent may include nitriles such as T-CN(wherein T is a hydrocarbon group having a C₂ to C₂₀ linear, branched,or cyclic structure and may include a double bond, an aromatic ring, oran ether bond), and/or the like, dioxolanes such as 1,3-dioxolane,and/or the like, sulfolanes, and/or the like.

The non-aqueous organic solvent may be used alone or in a mixture. Whenthe organic solvent is used in a mixture, the mixture ratio may becontrolled in accordance with a desirable battery performance.

The carbonate-based solvent is prepared by mixing a cyclic carbonate anda linear carbonate. When the cyclic carbonate and linear carbonate aremixed together in a volume ratio of 1:1 to 1:9, an electrolyteperformance may be improved.

The non-aqueous organic solvent may further include an aromatichydrocarbon-based organic solvent in addition to the carbonate-basedsolvent. Herein, the carbonate-based solvent and the aromatichydrocarbon-based organic solvent may be mixed in a volume ratio of 1:1to 30:1.

The aromatic hydrocarbon-based organic solvent may be an aromatichydrocarbon-based compound of Chemical Formula 2.

In Chemical Formula 2, R₁ to R₆ are the same or different and are eachindependently selected from the group consisting of hydrogen, halogen,C₁ to C₁₀ alkyl group, haloalkyl group, and a combination thereof.

Specific examples of the aromatic hydrocarbon-based organic solvent maybe selected from benzene, fluorobenzene, 1,2-difluorobenzene,1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene,1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene,1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene,1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene,1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene,1,2,4-triiodobenzene, toluene, fluorotoluene, 2,3-difluorotoluene,2,4-difluorotoluene, 2,5-difluorotoluene, 2,3,4-trifluorotoluene,2,3,5-trifluorotoluene, chlorotoluene, 2,3-dichlorotoluene,2,4-dichlorotoluene, 2,5-dichlorotoluene, 2,3,4-trichlorotoluene,2,3,5-trichlorotoluene, iodotoluene, 2,3-diiodotoluene,2,4-diiodotoluene, 2,5-diiodotoluene, 2,3,4-triiodotoluene,2,3,5-triiodotoluene, xylene, and a combination thereof.

The electrolyte of a lithium secondary battery may further include vinylethylene carbonate, vinylene carbonate, an ethylene carbonate-basedcompound represented by Chemical Formula 3, or a combination thereof, inorder to improve cycle life of a battery.

In Chemical Formula 3, R₇ and R₈ are each independently selected fromhydrogen, a halogen, a cyano group (CN), a nitro group (NO₂), and afluorinated C₁ to C₅ alkyl group, provided that at least one of R₇ andR₈ is selected from a halogen, a cyano group (CN), a nitro group (NO₂),and a fluorinated C₁ to C₅ alkyl group, and R₇ and R₈ are notsimultaneously hydrogen.

Examples of the ethylene carbonate-based compound may be difluoroethylene carbonate, chloroethylene carbonate, dichloroethylenecarbonate, bromoethylene carbonate, dibromoethylene carbonate,nitroethylene carbonate, cyanoethylene carbonate, fluoroethylenecarbonate, and the like. The amount of the additive for improving cyclelife may be used within an appropriate range.

The lithium salt dissolved in the non-aqueous solvent supplies lithiumions in a battery, enables a basic operation of a lithium secondarybattery, and improves transportation of the lithium ions betweenpositive and negative electrodes. Examples of the lithium salt includeat least one supporting salt selected from LiPF₆, LiSbF₆, LiAsF₆,LiN(SO₂C₂F₅)₂, Li(CF₃SO₂)₂N, LiN(SO₃C₂F₅)₂, LiC₄F₉SO₃, LiClO₄, LiAlO₂,LiAlCl₄, LiPO₂F₂, LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂) (wherein x andy are natural numbers, for example, an integer from 0 to 20, forexample, 1 to 20), lithium difluoro(bisoxolato) phosphate) (LiDFOP),LiCl, Lil, LiB(C₂O₄)₂ (lithium bis(oxalato) borate; LiBOB), and lithiumdifluoro(oxalate) borate (LiDFOB). The lithium salt may be used in aconcentration from 0.1 M to 2.0 M. When the lithium salt is included atthe above concentration range, an electrolyte may have excellentperformance and lithium ion mobility due to optimal electrolyteconductivity and viscosity.

FIG. 1 is a schematic view showing a structure of a lithium secondarybattery according to an embodiment of this disclosure.

Referring to FIG. 1, a lithium secondary battery 100 according to anembodiment may include an electrode assembly 40 and a case 50 in whichthe electrode assembly 40 is contained.

The electrode assembly 40 includes a positive electrode 10 including apositive active material, a negative electrode 20 including the negativeactive material, and a separator 30 disposed between the positiveelectrode 10 and the negative electrode 20. The positive electrode 10,the negative electrode 20, and the separator 30 may be impregnated inthe aforementioned electrolyte solution (not shown) according to thepresent disclosure.

The positive electrode 10 includes a current collector and a positiveactive material layer disposed on the current collector and including apositive active material.

In the positive active material layer, the positive active material mayinclude a compound (lithiated intercalation compound) being capable ofintercalating and deintercallating lithium and specifically at least onecomposite oxide of lithium and a metal of cobalt, manganese, nickel, anda combination thereof may be used. Specific examples thereof may be acompound represented by one of the following chemical formulae.Li_(a)A_(1-b)X_(b)D₂ (0.90≤a≤1.8, 0≤b≤0.5);Li_(a)A_(1-b)X_(b)O_(2-c)D_(c) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05);Li_(a)E_(1-b)X_(b)O_(2-c)D_(c) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05);Li_(a)E_(2-b)X_(b)O_(4-c)D_(c) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05);Li_(a)Ni_(1-b-c)Co_(b)X_(c)D_(α) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.5, 0<α≤2);Li_(a)Ni_(1-b-c)Co_(b)X_(c)O_(2-α)T_(α) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05,0<α<2); Li_(a)Ni_(1-b-c)Co_(b)X_(c)O_(2-α)T₂ (0.90≤a≤1.8, 0≤b≤0.5,0≤c≤0.05, 0<α<2); Li_(a)Ni_(1-b-c)Mn_(b)X_(c)D_(α) (0.90≤a≤1.8, 0≤b≤0.5,0≤c≤0.05, 0<α≤2); Li_(a)Ni_(1-b-c)Mn_(b)X_(c)O_(2-α)T_(α) (0.90≤a·1.8,0≤b≤0.5, 0≤c≤0.05, 0<α<2); Li_(a)Ni_(1-b-c)Mn_(b)X_(c)O_(2-α)T₂(0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α<2); Li_(a)Ni_(b)E_(c)G_(d)O₂(0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0.001≤d≤0.1);Li_(a)Ni_(b)Co_(c)Mn_(d)G_(e)O₂ (0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0≤d≤0.5,0.001≤e≤0.1); Li_(a)NiG_(b)O₂ (0.90≤a≤1.8, 0.001≤b≤0.1); Li_(a)CoG_(b)O₂(0.90≤a≤1.8, 0.001≤b≤0.1); Li_(a)Mn_(1-b)G_(b)O₂ (0.90≤a≤1.8,0.001≤b≤0.1); Li_(a)Mn₂G_(b)O₄ (0.90≤a≤1.8, 0.001≤b≤0.1);Li_(a)Mn_(1-g)G_(g)PO₄ (0.90≤a≤1.8, 0≤g≤0.5); QO₂; QS₂; LiQS₂; V₂O₅;LiV₂O₅; LiZO₂; LiNiVO₄; Li_((3-f))J₂(PO₄)₃(0≤f≤2);Li_((3-f))Fe₂(PO₄)₃(0≤f≤2); and Li_(a)FePO₄ (0.90≤a≤1.8).

In the chemical formulae, A is selected from Ni, Co, Mn, and acombination thereof; X is selected from Al, Ni, Co, Mn, Cr, Fe, Mg, Sr,V, a rare earth element, and a combination thereof; D is selected fromO, F, S, P, and a combination thereof; E is selected from Co, Mn, and acombination thereof; T is selected from F, S, P, and a combinationthereof; G is selected from Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, and acombination thereof; Q is selected from Ti, Mo, Mn, and a combinationthereof; Z is selected from Cr, V, Fe, Sc, Y, and a combination thereof;and J is selected from V, Cr, Mn, Co, Ni, Cu, and a combination thereof.

Particularly, in one embodiment, the positive active material maypreferably be a nickel-containing lithium transition metal compound(e.g., a nickel-included lithium transition metal compound) positiveactive material. For example, the positive active material may be ahigh-Ni positive active material having nickel in a content of 60 mol %or more in the nickel-containing lithium transition metal compound. Thatis, nickel may be included at 60 mol % (molar percent) or more based onthe total molar amount of all the transition metals included in thelithium transition metal compound. The high-Ni positive active materialmay be deteriorated (e.g., may cause the deterioration) due to Ni²⁺self-reduction and a side reaction with residual lithium (reaction ofmoisture and CO₂) on a surface of the positive electrode. However, theuse of the negative electrode with Si at an amount of about 0.1 wt % toabout 32 wt % based on 100 wt % of the total weight of the negativeactive material and the electrolyte with the additive including thecompound of Chemical Formula 1 may effectively prevent or substantiallyprevent these shortcomings. In contrast, the effects of using thenegative electrode and the electrolyte according to one or moreembodiments may not be significant when a cobalt-based active materialis used as the positive active material. In particular, it is notdesirable to use the electrolyte according to an embodiment in ahigh-voltage battery using the cobalt-based active material to partiallyoxidize a phosphorous-based film which may be generated on a surface ofthe positive electrode to decompose it. That is, the use of theelectrolyte according to one or more embodiments in a high-voltagebattery using the cobalt-based active material is not desirable, due tothe decomposition of a phosphorous-based film, which may be generated ona surface of the positive electrode, through partial oxidization.

According to an embodiment, the nickel-containing lithium transitionmetal compound positive active material may be one represented byChemical Formula 4.

Li_(p)(Ni_(x)Co_(y)Me_(z))O₂[  Chemical Formula 4]

In Chemical Formula 4, 0.9≤p≤1.1, 0.6≤x≤0.98, 0<y≤0.3, 0<z≤0.3, x+y+z 1,and Me is at least one of Al, Mn, Mg, Ti, and Zr.

More specifically, in Chemical Formula 4, x may be in the range of0.7≤x≤0.98.

The lithium metal oxide may have a coating layer on the surface, or maybe mixed with another lithium metal oxide having a coating layer. Thecoating layer may include at least one coating element compound selectedfrom an oxide of a coating element, a hydroxide of a coating element, anoxyhydroxide of a coating element, an oxycarbonate of a coating element,and a hydroxy carbonate of a coating element. The compound for thecoating layer may be amorphous or crystalline. The coating elementincluded in the coating layer may include Mg, Al, Co, K, Na, Ca, Si, Ti,V, Sn, Ge, Ga, B, As, Zr, or a mixture thereof. The coating layer may bedisposed by a method having no adverse influence on properties of apositive active material by using these elements in the compound, (e.g.,the method may include any coating method (e.g., spray coating, dipping,etc.)), but is not illustrated in more detail since it is well-known tothose skilled in the related field.

In the positive electrode, the positive active material may be includedin an amount of 90 wt % to 98 wt % based on a total weight of thepositive active material layer.

In an embodiment, the positive active material layer may include abinder and a conductive material. Herein, the binder and the conductivematerial may be included in an amount of 1 wt % to 5 wt %, respectivelybased on the total amount of the positive active material layer.

The binder serves to adhere the positive active material particles withone another and to adhere the positive active material to a currentcollector and examples thereof may be polyvinyl alcohol, carboxylmethylcellulose, hydroxypropyl cellulose, diacetyl cellulose,polyvinylchloride, carboxylated polyvinylchloride, polyvinylfluoride, anethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane,polytetrafluoroethylene, polyvinylidene fluoride, polyethylene,polypropylene, a styrene-butadiene rubber, an acrylatedstyrene-butadiene rubber, an epoxy resin, nylon, and the like, but arenot limited thereto.

The conductive material is included to provide conductivity to theelectrode and any electrically conductive material may be used as aconductive material unless it causes a chemical change. Examples of theconductive material may include a carbon-based material such as naturalgraphite, artificial graphite, carbon black, acetylene black, ketjenblack, carbon fiber, and/or the like; a metal-based material of a metalpowder or a metal fiber including copper, nickel, aluminum, silver,and/or the like; a conductive polymer such as a polyphenylenederivative; or a mixture thereof.

The current collector may be an aluminum foil, a nickel foil, or acombination thereof, but is not limited thereto.

The negative electrode 20 includes a current collector and a negativeactive material layer disposed on the current collector and including anegative active material.

The negative active material may include a material that reversiblyintercalates/deintercalates lithium ions, a lithium metal, a lithiummetal alloy, a material capable of doping/dedoping lithium, or atransition metal oxide.

The material that reversibly intercalates/deintercalates lithium ionsmay include for example, a carbon material, and i.e., the carbonmaterial which may be a generally-used carbon-based negative activematerial in a lithium secondary battery. Examples of the carbon-basednegative active material may include crystalline carbon, amorphouscarbon, or mixtures thereof. The crystalline carbon may be unspecifiedshaped, or sheet, flake, spherical, or fiber shaped natural graphite orartificial graphite. The amorphous carbon may be a soft carbon, a hardcarbon, a mesophase pitch carbonization product, fired coke, and/or thelike.

The lithium metal alloy includes an alloy of lithium and at least oneadditional metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si,Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn.

The material being capable of doping/dedoping lithium may be Si, SiO_(x)(0<x<2), a Si-Q alloy (wherein Q is an element selected from an alkalimetal, an alkaline-earth metal, a Group 13 element, a Group 14 element,a Group 15 element, a Group 16 element, a transition metal, a rare earthelement, and a combination thereof, and not Si), a Si-carbon composite,Sn, SnO₂, Sn—R (wherein R is an element selected from an alkali metal,an alkaline-earth metal, a Group 13 element, a Group 14 element, a Group15 element, a Group 16 element, a transition metal, a rare earthelement, and a combination thereof, and not Sn), a Sn-carbon composite,and/or the like. At least one of these materials may be mixed with SiO₂.The elements Q and R may each independently be selected from Mg, Ca, Sr,Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh,Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn,In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and a combination thereof.

The transition metal oxide may include lithium titanium oxide.

Particularly, in one embodiment, the negative active material maydesirably include Si at an amount of about 0.1 wt % to about 32 wt %based on 100 wt % of the total weight of the negative active material.As such, when the negative active material including Si at an amount ofthe above range, higher capacity may be achieved (e.g., exhibited).

The negative active material including Si at an amount of about 0.1 wt %to about 32 wt % may be i) a material including a core including amixture of crystalline carbon and Si, and an amorphous carbon coatinglayer surrounding the core, or ii) a material including a core includingcrystalline carbon and Si adhered to a surface of the core. That is, inone embodiment, the negative active material (including Si at an amountof about 0.1 wt % to about 32 wt %) may include a core and an amorphouscarbon coating layer surrounding the core, wherein the core includes amixture of crystalline carbon and Si. In another embodiment, thenegative active material (including Si at an amount of about 0.1 wt % toabout 32 wt %) may include a core and Si adhered to a surface of thecore, wherein the core includes crystalline carbon.

When the negative active material includes i) a material including acore including a mixture of crystalline carbon and Si, and an amorphouscarbon coating layer surrounding the core, the amount of crystallinecarbon may be about 38 wt % to about 94.9 wt % based on 100 wt % of thetotal weight of the negative active material, and the amount of theamorphous carbon may be about 5 wt % to about 30 wt %. Furthermore, theamorphous carbon coating layer may be substantially totally surroundedon the surface of the core, that is, may be a layer which may becontinuously presented on the surface of the core. For example, theamorphous carbon coating layer may be a continuously layer that coverssubstantially the whole surface of the core. In another embodiment, thecoating layer may be presented to partially expose the surface of thecore, that is, may be an island-type which may be discontinuouslypresented on the surface of the core. For example, the coating layer mayhave an island shape and forms a discontinuous layer on the surface ofthe core, which does not cover the whole surface of the core and exposesa portion of the surface of the core. When the negative active materialis ii) a material including a core including crystalline carbon and Siadhered to the surface of the core, the amount of the crystalline carbonmay be about 99.9 wt % to about 68 wt %. In addition, Si may bepresented on the surface of the core in the form of the island-typewhich is discontinuously presented to partially expose the surface ofthe core. For example, Si may be presented on the surface of the core inthe shape of islands and forms a discontinuous layer, which exposes aportion of the surface of the core.

The negative active material layer includes a negative active materialand a binder, and optionally a conductive material.

In the negative active material layer, the negative active material maybe included in an amount of 95 wt % to 99 wt % based on the total weightof the negative active material layer. In the negative active materiallayer, a content of the binder may be 1 wt % to 5 wt % based on a totalweight of the negative active material layer. When the negative activematerial layer includes a conductive material, the negative activematerial layer includes 90 wt % to 98 wt % of the negative activematerial, 1 wt % to 5 wt % of the binder, and 1 wt % to 5 wt % of theconductive material.

The binder improves binding properties of negative active materialparticles with one another and with a current collector. The binder mayuse a non-water-soluble binder, a water-soluble binder, or a combinationthereof.

The non-water-soluble binder may be polyvinylchloride, carboxylatedpolyvinylchloride, polyvinylfluoride, an ethylene oxide-containingpolymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene,polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide,polyimide, or a combination thereof.

The water-soluble binder may be a styrene-butadiene rubber, an acrylatedstyrene-butadiene rubber, polyvinyl alcohol, sodium polyacrylate, acopolymer of propylene and a C₂ to C₈ olefin, a copolymer of(meth)acrylic acid and (meth)acrylic acid alkyl ester, or a combinationthereof.

When the water-soluble binder is used as a negative electrode binder, acellulose-based compound may be further used to provide viscosity as athickener. The cellulose-based compound includes one or more ofcarboxymethyl cellulose, hydroxypropylmethyl cellulose, methylcellulose, or alkali metal salts thereof. The alkali metals may be Na,K, or Li. The thickener may be included in an amount of 0.1 parts byweight to 3 parts by weight based on 100 parts by weight of the negativeactive material.

The conductive material is included to provide electrode conductivityand any electrically conductive material may be used as a conductivematerial unless it causes a chemical change. Examples of the conductivematerial include a carbon-based material such as natural graphite,artificial graphite, carbon black, acetylene black, ketjen black, denkablack, a carbon fiber, and the like; a metal-based material of a metalpowder or a metal fiber including copper, nickel, aluminum silver, andthe like; a conductive polymer such as a polyphenylene derivative; or amixture thereof.

The current collector may include one selected from a copper foil, anickel foil, a stainless steel foil, a titanium foil, a nickel foam, acopper foam, a polymer substrate coated with a conductive metal, and acombination thereof.

The positive active material layer and the negative active materiallayer are formed by mixing an active material, a binder and optionally aconductive material in a solvent to prepare an active materialcomposition, and coating the active material composition on a currentcollector. The formation method of the active material layer is wellknown, and thus is not described in detail in the present disclosure.The solvent includes N-methylpyrrolidone and the like, but is notlimited thereto. When a water-soluble binder is used in the negativeactive material layer, a solvent used for preparing the negative activematerial composition may be water.

The separator 30 may include polyethylene, polypropylene, polyvinylidenefluoride, and multi-layers thereof such as a polyethylene/polypropylenedouble-layered separator, a polyethylene/polypropylene/polyethylenetriple-layered separator, or a polypropylene/polyethylene/polypropylenetriple-layered separator.

Hereinafter, the disclosure (e.g., the subject matters of thedisclosure) will be specifically examined (e.g., illustrated) throughExamples.

Example 1 (1) Positive Electrode and Negative Electrode

97.3 wt % of LiCoO₂ as a positive active material, 1.4 wt % ofpolyvinylidene fluoride as a binder, and 1.3 wt % of ketjen black as aconductive material were mixed and then, dispersed inN-methylpyrrolidone to prepare positive active material slurry. Thepositive active material slurry was coated on an aluminum foil and then,dried and compressed to manufacture a positive electrode.

98 wt % of graphite as a negative active material, 1 wt % ofpolyvinylidene fluoride as a binder, and 1 wt % of ketjen black as aconductive material were mixed and then, dispersed inN-methylpyrrolidone to prepare a negative active material layercomposition, and the negative active material layer composition wascoated on a copper foil and dried to manufacture a negative electrode.

(2) Preparation of Electrolyte

0.95 M LiPF₆ was added to a mixed solution of ethylene carbonate(EC):diethyl carbonate (DEC):ethylpropionate (EP) mixed in a volumeratio of 30:50:20 to prepare a non-aqueous mixed solution.

1.0 wt % of a compound represented by Chemical Formula 1-1 based on anamount of the non-aqueous mixed solution was added thereto tomanufacture an electrolyte for a lithium secondary battery.

(3) Manufacture of Lithium Secondary Battery Cell

The positive and negative electrodes manufactured according to the (1)and the electrolyte prepared according to the (2) were used tomanufacture a lithium secondary battery cell.

Example 2 (1) Manufacture of Positive Electrode and Negative Electrode

A positive electrode and a negative electrode were manufactured in thesame method as Example 1.

(2) Manufacture of Electrolyte

0.95 M LiPF₆ was added to a first mixed solution of ethylene carbonate(EC):ethylmethyl carbonate (EMC):ethylpropionate (EP):γ-butyrolactone(GBL) used in a volume ratio of 27:50:20:3 to prepare a second mixedsolution.

6 wt % of fluoroethylene carbonate (FEC), 0.5 wt % of vinyl ethylenecarbonate (VEC), 0.2 wt % of lithium tetrafluoroborate (LiBF₄), 5 wt %of succinonitrile (SN), 2 wt % of hexane tri-cyanide (HTCN), and 0.5 wt% of a compound represented by Chemical Formula 1-1 based on an amountof the second mixed solution were added thereto to prepare anelectrolyte for a lithium secondary battery.

(3) Manufacture of Lithium Secondary Battery Cell

A prismatic lithium secondary battery was manufactured by housing thepositive and negative electrodes and a polypropylene separator in acontainer and injecting the prepared electrolyte thereinto.

Example 3

A lithium secondary battery cell was manufactured according to the samemethod as Example 2 except that 0.25 wt % of the compound represented byChemical Formula 1-1 was added to prepare an electrolyte.

Example 4

A lithium secondary battery cell was manufactured according to the samemethod as Example 2 except that 1 wt % of the compound represented byChemical Formula 1-1 was added to prepare an electrolyte.

Example 5

A lithium secondary battery cell was manufactured according to the samemethod as Example 2 except that 2 wt % of the compound represented byChemical Formula 1-1 was added to prepare an electrolyte.

Comparative Example 1 (1) Manufacture of Positive Electrode and NegativeElectrode

A positive electrode and a negative electrode were manufacturedaccording to the same method as Example 1.

(2) Manufacture of Electrolyte

0.95 M LiPF₆ was added to a mixed solution of ethylene carbonate(EC):diethyl carbonate (DEC):ethylpropionate (EP) used in a volume ratioof 30:50:20 to prepare a non-aqueous mixed solution.

(3) Manufacture of Lithium Secondary Battery Cell

A prismatic lithium secondary battery cell was manufactured by housingthe positive and negative electrodes manufactured in the above (1) andthe electrolyte prepared in the above (2).

Comparative Example 2 (1) Manufacture of Positive Electrode and NegativeElectrode

A positive electrode and a negative electrode were manufacturedaccording to the same method as Example 1.

(2) Manufacture of Electrolyte

An electrolyte for a lithium secondary battery cell was manufacturedaccording to the same method as Example 2 except that the compoundrepresented by Chemical Formula 1-1 was not added thereto.

(3) Manufacture of Lithium Secondary Battery Cell

A lithium secondary battery cell was manufactured according to the samemethod as Example 2.

Experimental Example 1—CV Characteristics Evaluation

Cyclic voltammetry (CV) characteristics of the half-cells according toExample 1 and Comparative Example 1 were evaluated. The result ofExample 1 is shown in FIG. 2, and the result of Comparative Example 1 isshown in FIG. 3.

In FIGS. 2 and 3, 1, 3, and 5 denote the number of cycles.

Referring to FIGS. 2 and 3, currents were largely increased during bothof intercalation/deintercalation of lithium in FIG. 2 compared with FIG.3. Accordingly, when the lithium secondary battery cell including theelectrolyte using additives including the compound represented byChemical Formula 1-1 according to Example 1 was compared with thelithium secondary battery cell according to Comparative Example 1,lithium ions were relatively more easily intercalated/deintercalated.

Experimental Example 2—Evaluation of LSV Characteristics

The electrolytes of Example 1 and Comparative Example 1 were evaluatedregarding oxidation electrode decomposition at 25° C. using a linearsweep voltammetry (LSV) method. In this evaluation, a three-electrodeelectrochemical cell using a Pt electrode as a working electrode, acounter electrode, and Li as a reference electrode was used. Herein, ascan was performed in a range of 3.0 V to 7.0 V at a rate of 1 mV/sec,and the results are shown in FIGS. 4A and 4B. FIG. 4A shows LSVevaluation results of Example 1 and Comparative Example 1, and FIG. 4Bis an enlarged view showing a y-axis scale of FIG. 4A.

Referring to FIGS. 4A and 4B, the electrolyte including additivesincluding the compound represented by Chemical Formula 1-1 according toExample 1 exhibited an onset potential increase at a higher voltagecompared with the electrolyte according to Comparative Example 1.Accordingly, when the additive of Chemical Formula 1-1 was added,oxidation resistance of an electrolyte was increased.

Experimental Example 3—Thickness Increase Rate

The lithium secondary battery cells according to Example 2 andComparative Example 2 were constant current charged at a current densityof 1 C until a voltage reached 4.45 V. After measuring thicknesses ofthe cells after the charge, their thickness variation ratios (%) weremeasured every seven day, while stored at 60° C. for 28 days. Thethickness variation ratios measured at the 28th day were shown in Table1.

TABLE 1 Thickness Division increase rate (%) Example 2 14.3 Example 322.9 Example 4 9.0 Example 5 11 Comparative 23.8 Example 2

Referring to Table 1 and FIG. 5, the lithium secondary battery cellseach using the electrolyte including the additive of the presentinvention according to each of the example embodiments exhibited asignificantly reduced thickness compared with the lithium secondarybattery cells each not including the additive according to ComparativeExample.

Experimental Example 4—Cycle Characteristics

The lithium secondary battery cells according to Example 2 andComparative Example 2 were constant current charged at current densityof 1 C at 45° C. until a voltage reached 4.45 V. Subsequently, thelithium secondary battery cells were allowed to stand for 10 minutes andthen, discharged at constant current of 1 C, until the voltage reached 3V, and this charge and discharge as one cycle was 250 times repeated.The results are shown in FIG. 5.

Referring to FIG. 5, the lithium secondary battery cells according toExamples exhibited excellent high temperature cycle-life characteristicscompared with the lithium secondary battery cell according toComparative Example.

Example 6 (1) Positive Electrode and Negative Electrode

97.3 wt % of LiNi_(0.88)Co_(0.1)Al_(0.02)O₂ as a positive activematerial, 1.4 wt % of polyvinylidene fluoride as a binder, and 1.3 wt %of ketjen black as a conductive material were mixed and then dispersedin N-methylpyrrolidone to prepare a positive active material slurry. Thepositive active material slurry was coated on an aluminum foil and thendried and compressed to manufacture a positive electrode.

98 wt % of a negative active material including a graphite core and Siadhered to the surface of the core (graphite core at 99 wt % and Si at 1wt %), 1 wt % of polyvinylidene fluoride as a binder, and 1 wt % ofketjen black as a conductive material were mixed and then dispersed inN-methylpyrrolidone to prepare a negative active material layercomposition, and the negative active material layer composition wascoated on a copper foil and dried to manufacture a negative electrode.

(2) Preparation of Electrolyte

1.5 M LiPF₆ was added to a mixed solution of ethylene carbonate(EC):ethyl methyl carbonate (EMC):dimethyl carbonate (DMC) mixed in avolume ratio of 20:10:70 to prepare a non-aqueous mixed solution.

3.0 wt % of fluoroethylene carbonate and 0.5 wt % of a compoundrepresented by Chemical Formula 1-1 based on 100 wt % of the non-aqueousmixed solution were added thereto to manufacture an electrolyte for alithium secondary battery.

(3) Manufacture of Lithium Secondary Battery Cell

The positive and negative electrodes manufactured according to (1), theelectrolyte prepared according to (2), and a polypropylene coated withAl₂O₃ separator (CCS) were used to manufacture a lithium secondarybattery cell.

Example 7

A lithium secondary battery cell was manufactured according to the samemethod as Example 6, except that 1 wt % of the compound represented byChemical Formula 1-1 was added to prepare an electrolyte.

Example 8

A lithium secondary battery cell was manufactured according to the samemethod as Example 6, except that 2 wt % of the compound represented byChemical Formula 1-1 was added to prepare an electrolyte.

Example 9

A lithium secondary battery cell was manufactured according to the samemethod as Example 7, except that a negative active material including agraphite core and Si adhered to the surface of the core (graphite coreat 95 wt % and Si at 5 wt %) was used as a negative active material.

Example 10

A lithium secondary battery cell was manufactured according to the samemethod as Example 7, except that a negative active material including agraphite core and Si adhered to the surface of the core (graphite coreat 88 wt % and Si at 12 wt %) was used as a negative active material.

Example 11

A lithium secondary battery cell was manufactured according to the samemethod as Example 6, except that a negative active material including agraphite core and Si adhered to the surface of the core (graphite coreat 68 wt % and Si at 32 wt %) was used as a negative active material.

Example 12

A lithium secondary battery cell was manufactured according to the samemethod as Example 7, except that a negative active material including agraphite core and Si adhered to the surface of the core (graphite coreat 68 wt % and Si at 32 wt %) was used as a negative active material.

Example 13

A lithium secondary battery cell was manufactured according to the samemethod as Example 8, except that a negative active material including agraphite core and Si adhered to the surface of the core (graphite coreat 68 wt % and Si at 32 wt %) was used as a negative active material.

Example 14

A lithium secondary battery cell was manufactured according to the samemethod as Example 6, except that a negative active material including agraphite core and Si adhered to the surface of the core (graphite coreat 99.9 wt % and Si at 0.1 wt %) was used as a negative active material.

Example 15

A lithium secondary battery cell was manufactured according to the samemethod as Example 6, except that 0.1 wt % of the compound represented byChemical Formula 1-1 was added to prepare an electrolyte.

Reference Example 1 (1) Positive Electrode and Negative Electrode

A positive electrode was manufactured according to the same method asExample 6.

98 wt % of graphite as a negative active material, 1 wt % ofpolyvinylidene fluoride as a binder, and 1 wt % of ketjen black as aconductive material were mixed and then dispersed in N-methylpyrrolidoneto prepare a negative active material slurry. The negative activematerial slurry was coated on a copper foil, and then dried andcompressed to manufacture a negative electrode.

(2) Preparation of Electrolyte

1.15 M LiPF₆ was added to a mixed solution of ethylene carbonate (EC):ethyl methyl carbonate (EMC):dimethyl carbonate (DMC) mixed in a volumeratio of 20:10:60 to prepare a non-aqueous mixed solution. 3 wt % offluoroethylene carbonate and 0.05 wt % of a compound represented byChemical Formula 1-1 based on 100 wt % of the non-aqueous mixed solutionwere added thereto to manufacture an electrolyte for a lithium secondarybattery.

(3) Manufacture of Lithium Secondary Battery Cell

The positive and negative electrodes manufactured according to (1), theelectrolyte prepared according to (2), and a polypropylene coated withAl₂O₃ separator (CCS) were used to manufacture a lithium secondarybattery cell.

Reference Example 2

A lithium secondary battery cell was manufactured according to the samemethod as Example 7, except that a negative active material including agraphite core and Si adhered to the surface of the core (graphite coreat 50 wt % and Si at 50 wt %) was used as a negative active material.

Reference Example 3

A lithium secondary battery cell was manufactured according to the samemethod as Example 11, except that 5 wt % of the compound represented byChemical Formula 1-1 was added to prepare an electrolyte.

Reference Example 4

A lithium secondary battery cell was manufactured according to the samemethod as Example 15, except that a negative active material including agraphite core and Si adhered to the surface of the core (graphite coreat 99.95 wt % and Si at 0.05 wt %) was used as a negative activematerial.

Reference Example 5

A lithium secondary battery cell was manufactured according to the samemethod as Example 15, except that a negative active material including agraphite core and Si adhered to the surface of the core (graphite coreat 67 wt % and Si at 33 wt %) was used as a negative active material.

Reference Example 6

A lithium secondary battery cell was manufactured according to the samemethod as Reference Example 5, except that 0.05 wt % of the compoundrepresented by Chemical Formula 1-1 was added to prepare an electrolyte.

Reference Example 7

A lithium secondary battery cell was manufactured according to the samemethod as Reference Example 6, except that 4 wt % of the compoundrepresented by Chemical Formula 1-1 was added to prepare an electrolyte.

Experimental Example 5—Amount of the Generated Gas

The lithium secondary battery cells according to Examples 6 to 15 andReference Examples 1 to 7 were each stored at 60° C. for 30 days and theamount of the generated gas was measured. The results are shown in Table2.

Experimental Example 6—DC-IR (Direct Current Internal Resistance)

The lithium secondary battery cells according to Examples 6 to 15 andReference Examples 1 to 7 were each stored at 60° C. for 30 days and thedirect-current internal resistance was measure by measuring a voltagedrop (V) which occurred while a current was flowed at 1 C for 1 secondunder a SOC50 condition (charged to be 50% of charge capacity based on100% of the entire battery charge capacity, i.e., fully charged). Theresults are shown in Table 2.

TABLE 2 Increase Total amount rate of of generated resistance Amount ofSi Amount of a gas after after in negative compound of allowing toallowing to active Chemical stand at a stand at a material Formula 1-1high temper- high temper- (wt %) (wt %) ature (g/cc) ature (%) Example 61 0.5 0.066 143 Example 7 1 1 0.061 135 Example 8 1 2 0.052 129 Example9 5 1 0.063 142 Example 10 12 1 0.075 151 Example 11 32 0.5 0.120 169Example 12 32 1 0.110 161 Example 13 32 2 0.101 149 Example 14 0.1 0.50.067 145 Example 15 1 0.1 0.090 145 Reference 0 0.05 0.130 167 Example1 Reference 50 1 0.172 180 Example 2 Reference 32 5 0.190 159 Example 3Reference 0.05 0.1 0.101 150 Example 4 Reference 33 0.1 0.131 180Example 5 Reference 33 0.05 0.130 167 Example 6 Reference 33 4 0.120 169Example 7

As shown in Table 2, the lithium battery cells each including a negativeactive material including Si at an amount of about 1 wt % to about 32 wt% and an electrolyte including an additive compound of Chemical Formula1-1 according to Examples 6 to 15 each exhibited a surprisingly lowamount of generated gas and a low resistance increase athigh-temperature storage, compared to the lithium battery cellsincluding Si at a large amount of 50 wt % (e.g., Reference Example 2),even though the electrolyte includes the same additive compound.

Furthermore, Reference Examples 3 and 7, each including a large amountof the additive of 5 wt % or 4 wt %, respectively, Reference Examples 1and 6, each including a small amount of the additive of 0.05 wt %, andReference Examples 4 and 5, each including Si at an amount of 0.05 wt %or 33 wt %, respectively, exhibited abruptly increased amount ofgenerated gas and resistance increase at high-temperature storage.

As used herein, the term “substantially,” “about,” and similar terms areused as terms of approximation and not as terms of degree, and areintended to account for the inherent deviations in measured orcalculated values that would be recognized by those of ordinary skill inthe art.

Also, any numerical range recited herein is intended to include allsub-ranges of the same numerical precision subsumed within the recitedrange. For example, a range of “1.0 to 10.0” is intended to include allsubranges between (and including) the recited minimum value of 1.0 andthe recited maximum value of 10.0, that is, having a minimum value equalto or greater than 1.0 and a maximum value equal to or less than 10.0,such as, for example, 2.4 to 7.6. Any maximum numerical limitationrecited herein is intended to include all lower numerical limitationssubsumed therein and any minimum numerical limitation recited in thisspecification is intended to include all higher numerical limitationssubsumed therein. Accordingly, Applicant reserves the right to amendthis specification, including the claims, to expressly recite anysub-range subsumed within the ranges expressly recited herein.

While this invention has been described in connection with what ispresently considered to be practical example embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, and on the contrary, it is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims, and equivalents thereof.

DESCRIPTION OF SYMBOLS

-   -   100: secondary battery    -   10: positive electrode    -   20: negative electrode    -   30: separator    -   50: case

What is claimed is:
 1. A lithium secondary battery, comprising: apositive electrode comprising a positive active material; a negativeelectrode comprising a negative active material; and an electrolytecomprising a non-aqueous organic solvent, a lithium salt, and anadditive comprising a compound represented by Chemical Formula 1,wherein the negative active material comprises Si at about 0.1 wt % toabout 32 wt % in amount based on a total amount of the negative activematerial:

and wherein, in Chemical Formula 1, A is a substituted or unsubstitutedaliphatic chain or (—C₂H₄—O—C₂H₄—)n, and n is an integer from 1 to 10.2. The lithium secondary battery of claim 1, wherein in Chemical Formula1, A is a C₂ to C₂₀ hydrocarbon chain or (—C₂H₄—O—C₂H₄—)n, and n is aninteger from 1 to
 5. 3. The lithium secondary battery of claim 1,wherein the compound represented by Chemical Formula 1 is a compoundrepresented by Chemical Formula 1-1:


4. The lithium secondary battery of claim 1, wherein the additive is 0.1wt % to 3 wt % in amount based on a total amount of the electrolyte. 5.The lithium secondary battery of claim 1, wherein the additive is 0.15wt % to 2 wt % in amount based on a total amount of the electrolyte. 6.The lithium secondary battery of claim 1, wherein the negative activematerial comprises: i) a core comprising a mixture of crystalline carbonand Si, and an amorphous carbon coating layer surrounding the core; orii) a core comprising crystalline carbon and Si adhered to a surface ofthe core.
 7. The lithium secondary battery of claim 1, wherein theadditive further comprises an additional additive selected fromfluoroethylene carbonate, vinylene carbonate, vinyl ethylene carbonate,succinonitrile, hexane tricyanide, lithium tetrafluoroborate, andpropane sultone.
 8. The lithium secondary battery of claim 7, whereinthe additional additive is 0.1 wt % to 20 wt % in amount based on atotal amount of the electrolyte.
 9. The lithium secondary battery ofclaim 1, wherein the positive active material comprises anickel-containing lithium transition metal compound.
 10. The lithiumsecondary battery of claim 9, wherein nickel is about 60 mol % or morein amount based on 100 mol % of all transition metals in thenickel-containing lithium transition metal compound.
 11. The lithiumsecondary battery of claim 9, wherein the positive active materialcomprises a lithium metal oxide represented by Chemical Formula 4:Li_(p)(Ni_(x)Co_(y)Me_(z))O₂[  Chemical Formula 4] wherein, in ChemicalFormula 4, 0.9≤p≤1.1, 0.6≤x≤0.98, 0<y≤0.3, 0<z≤0.3, x+y+z=1, and Me isat least one selected from Al, Mn, Mg, Ti, and Zr.
 12. The lithiumsecondary battery of claim 6, wherein the negative active materialcomprises the core comprising a mixture of crystalline carbon and Si,and the amorphous carbon coating layer surrounding the core, and whereinthe amorphous carbon coating layer covers substantially the wholesurface of the core.
 13. The lithium secondary battery of claim 6,wherein the negative active material comprises the core comprisingcrystalline carbon and Si adhered to the surface of the core, andwherein silicon is in a shape of islands and forms a discontinuous layeron the surface of the core.
 14. The lithium secondary battery of claim1, wherein the lithium salt is selected from LiPF₆, LiSbF₆, LiAsF₆,LiN(SO₂C₂F₅)₂, Li(CF₃SO₂)₂N, LiN(SO₃C₂F₅)₂, LiC₄F₉SO₃, LiClO₄, LiAlO₂,LiAlCl₄, LiCl, Lil, LiB(C₂O₄)₂, LiDFOP, LiDFOB, LiPO₂F₂, andLiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂), and wherein x and y are eachindependently an integer from 0 to 20.